1. The Field of the Invention
This invention relates to pressure transducers for medical use and, more particularly, to a novel, disposable pressure transducer apparatus for use in the direct measurement and/or monitoring of human blood pressure.
2. The Prior Art
When diagnosing and treating various bodily ailments, medical personnel often find it desirable to measure and/or monitor a patient's blood pressure. For example, blood pressure measurement and monitoring are frequently employed with patients suffering from shock or cardiovascular ailments. Advantageously, by measuring and/or monitoring the blood pressure of these and other types of patients, medical personnel are better able to detect blood flow difficulties and other cardiovascular problems at an early stage. As a result, the use of blood pressure measurement and monitoring may increase the likelihood that a patient can be successfully treated and/or provided with needed emergency assistance.
A variety of methods are currently used for measuring and/or monitoring blood pressure. For example, medical personnel frequently use various indirect blood pressure measurement techniques, such as measuring a patient's blood pressure by using a pressure cuff and a stethoscope. In addition, blood pressure measurements are often made using a number of direct measurement and monitoring techniques. Notably, when diagnosing and/or treating critically ill patients, such direct techniques are greatly preferred over any of the indirect techniques.
This preference for direct blood pressure measurement and monitoring in certain cases is due to several factors. First, the use of direct blood pressure measurement and monitoring greatly increases the accuracy of the blood pressure reading. Typical indirect techniques may, for example, yield errors as high as ten percent, whereas direct blood pressure measurement and monitoring techniques are generally accurate to within about one percent. In addition, direct monitoring techniques facilitate the continuous monitoring of a patient's blood pressure on a beat-to-beat basis. Direct blood pressure monitoring also enables the rapid detection of a change in cardiovascular activity, and this may be of significant importance in emergency situations. Further, direct blood pressure monitoring techniques can be readily used to measure and monitor a patient's blood pressure at a specific internal location, such as within the chambers of the heart. Because of these and other advantages, therefore, direct blood pressure measurement and monitoring has become part of the routine treatment for critically ill patients.
One of the most widely used techniques for direct blood pressure measurement and monitoring is called catheterization. In using this technique, a needle is first inserted into a peripheral blood vessel. For example, if it is desired to monitor arterial blood pressure, the needle may be inserted into the radial artery. If, on the other hand, venous blood pressure is to be monitored, the needle may be inserted into the antecubital, radial, jugular, or subclavian veins.
Once the needle is properly inserted, a special catheter is threaded through the needle and into the blood vessel. Importantly, this catheter is filled with some type of solution, such as, for example, a sterile saline solution. In addition, the catheter may be formed so as to facilitate the further threading of the catheter along the blood vessel. Thus, the catheter may be threaded through the needle and along the blood vessel until the tip of the catheter, which is located inside the blood vessel, is positioned at the particular point within the body at which it is desired to make the blood pressure measurement. Then, with the catheter thus in place, the needle may be withdrawn.
After the indwelling catheter is positioned within a patient as described above, the other end of the catheter is connected to pressure transducer. The catheter is generally also connected to a suitable continuous flush device or heparin drip to help prevent clotting around the tip of the catheter. The present transducer is then connected to some type of monitor device near the patient's bedside. Typical monitor devices include cathode ray tube display devices, digital display and/or recording devices, printers and plotters.
With the measurement equipment having been prepared for use in the above-described manner, any air bubbles within the catheter are next removed such that a continuous fluid column is provided from the pressure transducer to the tip of the catheter which is located within the patient's blood vessel. Then, as the patient's heart thereafter pumps blood, periodic pressure pulses are transmitted through the patient's blood vessels and along the fluid column in the catheter to the pressure transducer. The pressure transducer generates electrical signals representing the pressure pulses, and such signals are then amplified and displayed by the monitor device. Usually, the monitor device is used to display the patient's blood pressure as a function of time, this type of display being commonly referred to as the blood pressure waveform. A patient's blood pressure waveform can then be used by medical personnel to appropriately diagnose and treat the patient.
It will be readily appreciated that one of the most important components of the above-described blood pressure monitoring system is the pressure transducer. Significantly, the accuracy and reliability of the pressure transducer set an upper limit to the quality of the blood pressure data which can be obtained. Therefore, those skilled in the art of blood pressure monitoring have attempted to develop pressure transducers which have a high degree of reliability, sensitivity, and accuracy.
A typical pressure transducer for use in blood pressure monitoring systems comprises a thin diaphragm which is capable of being deflected by the pressure pulses which travel through the above-described fluid column in the catheter. Some type of mechanism is also provided for measuring the deflection of the diaphragm; and such a measuring mechanism usually comprises suitable electronic circuitry which is configured so as to generate an electrical signal representing the pressure exerted on the diaphragm.
While a variety of electronic mechanisms have been used to measure the deflection of a diaphragm in pressure transducers, perhaps the most common measuring mechanism which is currently in use comprises a resistive strain gauge, such a mechanism being quite similar to strain gauges that are commonly used in industrial applications. Basically, a resistive strain gauge comprises a thin resistive wire which is connected to the pressure diaphragm such that the wire is stretched whenever the diaphragm is deflected. In accordance with well-known principles, such a stretching of the wire causes the electrical resistance of the wire to increase. Assuming, therefore, that a constant voltage is being applied across the wire, such an increase in the wire's resistance will result in a corresponding decrease in the electrical current through the wire in accordance with Ohm's law. Thus, by continuously measuring the current through the wire, it is possible to obtain an electrical signal which represents the amount by which the diaphragm is being deflected and which, therefore, also represents the pressure being exerted on the diaphragm.
In order to increase the sensitivity and accuracy of the pressure measurement, it is common to connect four such resistive wires to a single pressure diaphragm. Typically, the wires are also connected together in a conventional Wheatstone bridge configuration. Moreover, two of the wires are connected to the diaphragm so as to be stretched when the diaphragm is deflected, while the other two wires are compressed as the diaphragm is deflected. Significantly, using this type of diaphragm/circuitry configuration, it is possible to obtain quite accurate measurements of even small pressure pulses acting on the pressure diaphragm.
Unfortunately, the above-described pressure transducers have typically been quite expensive to manufacture. Consequently, these transducers have generally been provided in the form of a reusable instrument which can be connected to a fluid-filled catheter by means of a disposable dome.Although such reusable pressure transducers can produce acceptable results, the use of such transducers has a number of significant disadvantages. First, since the transducer is to be used by a number of patients, it must be sterilized after each use; and this sterilization procedure can be both time consuming and expensive. Further, the repeated use and the uncertain life of the pressure transducer have made it difficult to accurately charge patients for the use of the transducer device. In addition, the transducer must undergo periodic maintenance in order to assure its proper functioning and operation. All of these factors have made the use of reusable pressure transducers somewhat burdensome and inconvenient.
With the growth of the semiconductor industry and the recent development of semiconductor pressure transducers, many of the above-mentioned problems of reusable transducers have been overcome. Using current semiconductor technology, it is now possible to provide the several required resistive elements on the surface of a single, bonded silicon chip. For example, these resistive elements may be implanted on a silicon chip used conventional ion implanting techniques. In addition, a portion of the chip may be etched away (such as, for example, by means of suitable chemicals), so as to form a thin pressure diaphragm. Thus, a single silicon chip can be formed to comprise both the pressure diaphragm and the measuring circuitry of a pressure transducer. Significantly, since such silicon chips are readily adapted to being mass produced, the total cost of pressure transducers can be reduced substantially. As a result of these developments, therefore, several manufacturers have recently placed disposable pressure transducers on the market.
Despite the advantages of using semiconductor chips in pressure transducers, however, the use of semiconductor transducers raises a number of additional problems which must be addressed. First, the output of semiconductor pressure transducers is typically temperature-dependent. Also, semiconductors (particularly silicon semiconductors), are quite sensitive to light, such that exposure of semiconductor pressure transducers to light can produce transient variations in the pressure readings. Additionally, as with other prior art pressure transducers, the pressure diaphragm and circuitry must be mounted such that thermal expansion and contraction of the diaphragm and circuitry will not subject the sensitive measuring circuitry to mechanical stress. Further, care must be taken to insure that the pressure transducer device is electrically isolated from the patient so that an electric shock will not be transmitted through the fluid column in the catheter and back to the patient. Some efforts have been made to solve each of these problems in the prior art semiconductor transducer devices, and these efforts are briefly discussed below.
In order to compensate for the temperature dependence of semiconductor pressure transducers, the prior art devices have typically included some type of temperature compensation circuitry. Such circuitry may comprise resistors, thermistors, or other electrical components which are designed to compensate for fluctuations in the transducer output which are caused by variations in temperature. In the various prior art devices, such temperature compensation circuitry may be located in a number of positions relative to the pressure diaphragm. For example, one prior art device provides the temperature compensation circuitry as part of a connector in a cable which is used to connect the transducer device to an external monitor.
The prior art devices have also attempted to compensate for the light sensitivity of semiconductors in a number of ways. For example, some prior art devices include a coating on the semiconductor transducer which is intended to shield the transducer from exposure to light. The prior art devices may additionally comprise some type of tinted plastic housing which surrounds the transducer. Such a tinted housing is also intended to shield the semiconductor transducer from light.
In addition, in order to minimize the difficulty caused by thermal expansion and contraction of the semiconductor transducer, prior art devices have typically mounted the transducer on a pyrex pedestal. Notably, pyrex has approximately the same coefficient of thermal expansion as silicon. Thus, the pyrex pedestal will expand and contract with variations in temperature in substantially the same manner as in silicon chip, thereby minimizing mechanical stress which may be caused by such thermal expansion and contraction.
Finally, electrical isolation of the prior art devices from a patient has typically been accomplished by various insulators. Such insulators may, for example, comprise part of a housing within which the electronic circuitry is contained.
While the prior art disposable pressure transducers represent an important improvement over the earlier reusable transducers, a number of significant difficulties remain. First, the the prior art disposable pressure transducers are not entirely effective in shielding the semiconductor transducer from exposure to light. As a result, the prior art devices sometimes produce incorrect transient pressure readings which may interfere with the subsequent analysis and use of the pressure data. In addition, a movement of the prior art devices will occasionally subject the sensitive pressure measuring circuitry to mechanical stress, thereby also producing transient fluctuations in the pressure readings. Because of these conditions, therefore, the prior art disposable pressure transducer devices will sometimes not produce data having the desired degree of accuracy.
Additionally, the electrical insulators in the prior art transducer devices have occasionally failed. When this happens, an electrical shock may be transmitted from the transducer device to the patient along the fluid-filled catheter. This, of course, represents a significant safety risk to the patient, particularly if the patient is already suffering from some serious illness or injury.
Further, the prior art disposable pressure transducers are typically quite complex, and they remain, therefore, relatively expensive to manufacture. For example, a number of separate, delicate assembly steps are often required in order to arrange the various components of the transducer devices into a single unit. Such assembly steps may, for example, include mounting a semiconductor transducer on a pyrex pedestal and thereafter securing the pedestal within a housing. Of course, the expense of manufacturing the prior art transducer devices increases the ultimate cost of the devices; and such increased cost makes it less desirable to dispose of the devices after only a single use.